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Composite membranes based on porous support membranes filled with a proton-conducting polymer appear to be a promising approach to develop novel proton exchange membranes (PEMs). It allows optimization of the properties of the filler and the matrix separately, e.g. for maximal conductivity of the former and maximal physical strength of the latter. In addition, the confinement itself can alter the properties of the filling ionomer, e.g. toward higher conductivity and selectivity due to alignment and restricted swelling. This article reviews the literature on PEMs prepared by filling of submicron and nanometric size pores with Nafion and other proton-conductive polymers. PEMs based on alternating perfluorinated and non-perfluorinated polymer systems and incorporation of fillers are briefly discussed too, as they share some structure/transport relationships with the pore-filling PEMs. We also review here the background knowledge on structural and transport properties of Nafion and proton-conducting polymers in general, as well as experimental methods concerned with preparation and characterization of pore-filling membranes. Such information will be useful for preparing next-generation composite membranes, which will allow maximal utilization of beneficial characteristics of polymeric proton conductors and understanding the complicated structure/transport relationships in the pore-filling composite PEMs. [ABSTRACT FROM AUTHOR]

The high proton conductivity of Nafion© membranes in direct methanol fuel cells is attributed to a random three dimensional 3D network of elongated ion channels formed by bundles of rod-like inverted micelles. However, this network is also responsible for high methanol crossover. In addition, its conductivity is impaired at temperatures 120 °C and above due to ion channel rearrangement, thus limiting the operational temperature to 60-80 °C. Here we report a composite proton exchange membrane prepared by filling the pores of polycarbonate track-etched membrane with Nafion, which promote channel orientation roughly normal to the surface. The proton conductivity of this geometry is significantly enhanced, presumably due to the restricted swelling, resulting in a better ion channel connectivity and realignment in the axial pore direction upon hydration. The permeability of sodium chloride and methanol, though, is suppressed, presumably due to formation of regions of closely packed sulfonic groups, which facilitate proton transfer but strongly exclude other solutes. Composite membranes with improved conductivity and selectivity based on the principle presented here are expected to allow operating direct methanol fuel cells at higher temperatures and with higher methanol concentrations, thus reducing costs and increasing the efficiency. [ABSTRACT FROM AUTHOR]